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Q090FreeSystemVerilog

Increment a four-digit BCD counter

Difficulty
Medium
Topic
RTL Design
Language
SV
Interview prompt

Question

Accept four valid BCD digits and add one without converting the packed value to binary. Return the result through a ready/valid interface that supports stalls. Wrap 9999 to 0000 and flag the wrap.

Candidate starting point

Implementation scaffold

module bcd_incrementer (
  input  logic       clk,
  input  logic       rst_n,
  input  logic       in_valid,
  output logic       in_ready,
  input  logic [3:0] d3, d2, d1, d0,
  output logic       out_valid,
  input  logic       out_ready,
  output logic [3:0] q3, q2, q1, q0,
  output logic       carry_out
);
  logic [3:0] next3, next2, next1, next0;
  logic next_carry;

  always_comb begin : decimal_successor
    // TODO: Implement decimal_successor using the supplied state and interface.
  end

  assign in_ready = !out_valid || out_ready;

  always_ff @(posedge clk) begin : elastic_response
    // TODO: Implement elastic_response using the supplied state and interface.
  end
endmodule
Reviewed example

Trace one case

Input
accepted BCD values 1999, then 9999; stall the second response
Expected output
1999 -> 2000 carry_out=0; 9999 -> 0000 carry_out=1, held stable while stalled

Decimal carry propagates across trailing nines, and the all-nine case wraps all four digits with one carry flag.

What to cover

Requirements

  1. Treat each input digit as a decimal value from 0 through 9.
  2. Propagate decimal carry across any run of trailing nines.
  3. Create exactly one output transaction for every accepted input transaction.
  4. Hold all result digits and carry_out stable while out_valid is asserted and out_ready is low.
Continue practicing
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